 /*******************************************************************************
  * Copyright (c) 2005, 2007 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  * Brad Reynolds - bug 164653
  *******************************************************************************/
 package org.eclipse.core.databinding.observable.value;

 import org.eclipse.core.databinding.observable.Diffs;
 import org.eclipse.core.databinding.observable.Realm;

 /**
  *
  * <p>
  * This class is thread safe. All state accessing methods must be invoked from
  * the {@link Realm#isCurrent() current realm}. Methods for adding and removing
  * listeners may be invoked from any thread.
  * </p>
  * @since 1.0
  *
  */
 public abstract class AbstractVetoableValue extends AbstractObservableValue
         implements IVetoableValue {

     /**
      * Creates a new vetoable value.
      */
     public AbstractVetoableValue() {
         this(Realm.getDefault());
     }

     /**
      * @param realm
      */
     public AbstractVetoableValue(Realm realm) {
         super(realm);
     }

     final protected void doSetValue(Object value) {
         Object currentValue = doGetValue();
         ValueDiff diff = Diffs.createValueDiff(currentValue, value);
         boolean okToProceed = fireValueChanging(diff);
         if (!okToProceed) {
             throw new ChangeVetoException("Change not permitted"); //$NON-NLS-1$
 }
         doSetApprovedValue(value);
         fireValueChange(diff);
     }

     /**
      * Sets the value. Invoked after performing veto checks.
      *
      * @param value
      */
     protected abstract void doSetApprovedValue(Object value);

     public synchronized void addValueChangingListener(
             IValueChangingListener listener) {
         addListener(ValueChangingEvent.TYPE, listener);
     }

     public synchronized void removeValueChangingListener(
             IValueChangingListener listener) {
         removeListener(ValueChangingEvent.TYPE, listener);
     }

     /**
      * Notifies listeners about a pending change, and returns true if no
      * listener vetoed the change.
      *
      * @param diff
      * @return false if the change was vetoed, true otherwise
      */
     protected boolean fireValueChanging(ValueDiff diff) {
         checkRealm();

         ValueChangingEvent event = new ValueChangingEvent(this, diff);
         fireEvent(event);
         return !event.veto;
     }

     public synchronized void dispose() {
         super.dispose();
     }

 }

